1. Field of the Invention
The present invention relates to a power supply device connected to a load circuit including a DC power supply.
2. Description of the Related Art
This type of conventional power supply device as disclosed in, for example, Patent document 1 includes: a first DC power supply; a chopper circuit made up of a chopper switching element Q1, synchronous-rectification switching element Q2 and reactor L1 connected to the connection point between the switching elements Q1 and Q2; an output smoothing capacitor Cout; and a drive circuit that controls turning on and off the chopper switching element Q1. The drive circuit outputs a drive signal in such a way as to turn on the switching element Q1 for a predetermined time T1 and turn off for a predetermined time T2 in an alternating manner, whereby input voltage from the first DC power supply is descended to a predetermined output voltage, and current is supplied to a load circuit connected in parallel to the output smoothing capacitor Cout.
Moreover, as shown in FIG. 8 of Patent document 2, for example, a filter circuit including a filter coil and filter capacitor is provided to prevent conducted noise made by a chopper circuit from flowing into a DC power supply or a load circuit.    Patent document 1: Japanese Laid-Open Patent Publication No. 2012-75207    Patent document 2: Japanese Laid-Open Patent Publication No. 2009-118552
In the foregoing conventional power supply devices, the reactor L1 is designed so as to obtain a predetermined inductance value when the rated current flows there. However, when the load circuit is configured including a second DC power supply, if the on-time T1 and off-time T2 are deviated from the values originally required, due to external disturbances such as load fluctuation of the load circuit, there would be a danger of an excessive current occurring.
In general, a reactor inherently has drooping characteristics in which its inductance value decreases when DC current flows therethrough; therefore once an excessive current flows, the current increasing rate goes up at an accelerated pace, which will likely lead to damage to the power supply device.
Therefore, a certain level of inductance value needs to be secured even in an excessive current flowing region, which has caused a problem in that the reactor will be increased in size.